In the context of this invention, the term "composite" denotes a material consisting essentially of high strength fibers or filaments of graphite or other material embedded in a matrix of a thermosetting resin which serves when cured to maintain the alignment of the fibers and their relationship to one another within the matrix as the material is stressed. As applied to the construction of aerodynamic surfaces, composite tapes are resin tapes embedded with fibers or filaments aligned in the longitudinal direction of the tape, multiple courses of which are laid side by side, using a tape head of the general kind to which this invention relates, to construct one ply or layer of a manufactured article, to which successive layers of tape are added incrementally in similar fashion.
The application of this construction technique to contoured surfaces is essentially one of first laying up the laminated structure on a mold having the final surface contour of the part to be fabricated so that the layup will assume the desired shape. The mold with the composite layup applied thereto is then autoclaved. The layers of resinous matrix material merge into a unitary structure during the initial stages of the process and then solidify upon continued exposure to the high temperature in the autoclave as the resin cures.
Upon removal from the mold, that part may be trimmed by machining and otherwise readied for assembly with other parts into an air frame or the like.
As explained in U.S. Pat. No. 4,696,707, Lewis et al., the layup of composite tape upon surfaces of compound curvature is preferably done by determining the natural path the tape would take in being rolled across the lay surface in the direction selected by the part designer for the desired orientation of the fibers embedded in the tape. That patent outlines the technique and a useful algorithm for pre-calculating the natural paths that would be taken by the tape on each course of each layer in the part buildup, working from a mathematical representation of the surface. The pre-calculated path or paths are then converted by appropriate means into machine commands for the coordinated movement of the tape head over the lay surface under numerical control of the multiple machine axes.
In an incremental construction process as described, it is inevitable that manufacturing tolerances, in the tape, in its laying or in the lay surface, will occur and may accumulate in a manner neither uniform nor precisely predictable, to cause deviations of the actual lay surface from the calculated or ideal mathematical representation, particularly in subsequent layers of the layup. This circumstance dictated the further utilization of adaptive control, in addition to programmed numerical control, for the movement of the tape laying head on certain ones of its multiple axes of movement. Such adaptive or reactive control of the tape head, as an adjunct to programmed numerically controlled movement along the pre-calculated natural tape path, is the subject of my prior U.S. Pat. No. 4,750,965 with James J. Pippel as co-inventor.
Inasmuch as the tape head of this invention retains much of the structure and all of the operational capability of the tape head of my prior U.S. Pat. No. 4,750,965 with James J. Pippel, the disclosure of that patent is hereby incorporated by reference as the point of departure for the present invention.
Inasmuch as the important differences between the tape laying functions of the tape head of this invention and that of U.S. Pat. No. 4,750,965 reside in the tape head proper and to some extent in control, a familiarity of the person skilled in this art will be assumed as to tape-laying generally, and detailed description of the invention centered on construction, operation, and method of utilization of the improved tape laying head proper.
It will be understood that the tape head of this invention, like that of U.S. Pat. No. 4,750,965, is movable through the operating space of a tape laying machine along three mutually perpendicular linear axes, X, Y, and Z, the X and Y axes being typically horizontal and the Z-axis vertical.
These linear motions are effected by a gantry arrangement in which the tape head is suspended from a ram vertically movable in a carriage mounted for horizontal movement on the cross rails of a larger carriage movable horizontally in a direction perpendicular to the direction of movement of the lesser carriage which it carries.
The vertical movement of the ram lifts or lowers the tape head away from or toward a formed mandrel positioned within the operating zone of the machine, and, in accordance with machine tool convention, is termed Z-axis movement. Horizontal movement of the tape head by movement of the ram carriage upon the larger carriage is termed Y-axis movement, while movement of the tape head by movement of the larger gantry carriage along its fixed support rails is termed X-axis movement.
These linear movements are driven by gear-and-rack and ball screw drives, with well-known provision where necessary to eliminate backlash, from servomotors with position and velocity feedback to the controller.
In addition to its linear motions, the tape head is movable about two rotational axes.
Alignment of the tape head, suspended as a whole from the lower end of the ram, in the direction of linear travel is effected by rotation on the axis of the ram. This rotation, termed C-axis rotation, is likewise gear-driven, with anti-backlash provision, from a servomotor, with appropriate position feedback from a resolver or the like incorporated with the drive motor.
The second rotational movement, termed A-axis movement and accommodated within the tape head itself, is a sideward rocking movement of the tape head about a horizontal axis perpendicular to the C-axis, and tangent to the underside of the laydown rollers in the vertical mid-plane of the tape head. Rotation of the tape head about the A-axis maintains the axes of the laydown rollers parallel to the lay surface when the tape head is laying tape on a side hill course.
In the arrangement of U.S. Pat. No. 4,750,965, the Z-axis, the C-axis, and the A-axis were each provided with adaptive control in addition to their programmed control in order to deal with unanticipated deviation of the actual lay surface from the numerically defined surface for or from which the programmed path was calculated. Adaptive control of the C-axis, however, was not sufficiently responsive in all circumstances due to the necessity of wielding the mass of the entire tape head, and in some degree to the divergence of the central plane of the laydown rollers from coincidence with the C-axis during side hill layment, so that adaptive rotation about the C-axis induced an adaptive adjustment about the A-axis and resulted in occasional undesirable hunting.
The tape head of U.S. Pat. No. 4,750,965 was also incapable of laying composite tape along other than its natural path, namely, on courses requiring curved paths, because it had no provision for effecting the relative longitudinal displacement of laterally adjacent incremental tape strands that is necessary for laying the inextensible tape in a curved path without puckering or wrinkling.
These operational difficulties and limitations have been largely obviated by the present invention.